Roland System 700 Analog Synthesizer Modules
5. The Low Frequency Oscillator (LFO)
The LFO is a module which is basically a VCO designed so that it only produces very low frequencies (up to c. 30 cps.). As such it is not intended to produce an audio signal, but rather a cyclic voltage output which is to be used as a "controller" of other modules on the synthesizer. Most typical is the use of an LFO to control a VCO, producing vibrato or trills, and, to a lessor degree, to control a VCF, where it might produce a cyclic filter sweep or effect. It might also be used to gate an ADSR but this is not as common, especially with currently available computer control possibilities.
Shown on the following drawing are the physical features of the Roland LFO:
Wave Form Select. This slider switch controls which of the four waveforms is produced by the LFO. Note that the pulse wave is, in this case, a square wave and that both the square and sawtooth wave forms are positive going.
Frequency Control. Adjusts the frequency of the LFO from c. 0.01 cps. to 30 cps..
Output. Jack which allows for the LFO to be patched into whatever module it is controlling. Note that on the main module LFO-1 is hardwired into the control inputs of various modules (VCOs and VCFs).
Control Input. This jack allows for the input of a controlling voltage that would affect the frequency of the LFO. Another LFO, for example, might be used to cyclically change the frequency of the main LFO.
Key Trigger Switch. When in the "on" position an incoming gate voltage will force the LFO to start the wave it is producing at the beginning of its cycle. This would mean that when a key is depressed the wave would automatically be locked in phase. With the switch in the "off" position the LFO's wave phase would be where ever it happened to be at that moment.
Delay Time Control. Restricts the output of the LFO from appearing for a set amount of time. Frequently used when the LFO is being employed to create vibrato where it emulates a performer's tendency to attack a note as a pure pitch and later add vibrato.
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